This invention generally pertains to an injection molding apparatus. More specifically, the present invention relates to a gas injection pin for such an injection molding apparatus.
The invention is particularly applicable to a gas assisted injection molding process in which a nozzle is utilized to inject a viscous fluid such as a molten plastic into a mold cavity and a non-viscous fluid such as a gas is also injected into the mold cavity. However, it will be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments where both a relatively viscous fluid, such as a plastic or wax, and a relatively non-viscous fluid such as a gas or liquid are injected into a mold cavity.
Injection molding processes have been widely known, not only for the production of molded articles made of various thermoplastic resins, but also for the production of lost wax masters used in the investment casting process.
One such known process is solid injection molding which employs the steps of injecting a plasticized (melted) thermoplastic material under high pressure into a finite mold space and then cooling the material sufficiently so that it rehardens to the extent that it can retain its shape after removal from the mold. Thermoplastic materials, generally shrink during rehardening and, unfortunately, this shrinkage is exaggerated in heavier wall sections, bosses, ribs, gussets, etc. This usually results in sink marks and warpage in the molded products.
Packing the mold with more material by pressing the plastic material at a higher pressure into the mold is a common technique used to minimize such excessive shrinkage. However, packing builds internal stresses into the part and often cannot remove sink marks that are located away from the injection molding sprue or gate. Additionally, packing requires high clamp pressures between the parts of the mold body in order to prevent flashing of the plastic material.
Certain proposals have recently been made to fill the mold cavity with a plasticized thermoplastic material to a volume less than one hundred percent (100%) of the mold space and to utilize an inert gas injected under pressure into the partially plasticized material as it is cooling and rehardening to fill the rest of the volume in the mold cavity. The gas enters the part and moves along the paths of least resistance therein. Such paths are normally in areas where the thermoplastic body is thicker and has slower cooling sections, such as ribs, flow channels, chamfers, etc. In this way, with a suitably designed part, a continuous network of hollowed out sections can be provided. The material displaced by the gas from the middle of the sections moves out to fill the remainder of the mold space. This network of gas channels provides a uniform pressure distribution system throughout the mold space during part rehardening and cool down, thus minimizing internal stresses. The outer surfaces of thicker sections do not sink because gas has cored them out from the inside and gas pressure holds the plastic material up against the mold surfaces during rehardening. Sink in these sections takes place internally rather than on the exterior surfaces of the part. Since the pressure used for final filling of the part is confined to an area defined by the gas channels, the resultant force against the sections of the mold is relatively modest so that lower clamping forces on the mold are adequate.
Most of the nozzles which are adapted to inject both a viscous fluid such as a thermoplastic material and a non-viscous fluid such as a gas into a mold cavity inject both of these fluids through a common nozzle. However, for the production of some moldings, especially moldings of a complex design, it is desirable to introduce the pressurized gas or other relatively non-viscous fluid at a different location than at which the plastic is introduced, or perhaps at several locations which are all spaced from the plastic injection point. Such a situation may arise, for example when it is desired to employ a hot runner system or the choice of gate positions is restricted by the desire to avoid an overabundance of gas channels which need to be connected to one another so that the injection of gas at one point will enable the gas to push the plastic out against the surfaces of the mold cavity throughout the extent of the mold cavity.
Accordingly, it has been considered desirable to develop a new and improved injection molding nozzle which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.